This invention relates generally to the production of chlorine dioxide. More particularly, it relates to an electrolytic generator used to manufacture chlorine-free chlorine dioxide from alkali metal chlorite solutions.
Chlorine dioxide is commercially employed as a bleaching, fumigating, sanitizing or sterilizing agent. Chlorine dioxide can be used to replace the chlorine and hypochlorite products more traditionally used in such applications with resultant benefits. Chlorine dioxide is a more powerful sterilizing agent and requires lower dose levels than chlorine at both low pHs and high pHs, although it is not particularly stable at high pH levels. More importantly, chlorine dioxide produces lower levels of chlorinated organic compounds than chlorine when it is used to sterilize raw water containing organic compounds. Additionally, chlorine dioxide is less corrosive than chlorine to metals.
The electrolytic production of chlorine dioxide is old and well known. See U.S. Pat. No. 2,163,793 issued Jun. 27, 1939 (J. O. Logan); British Patent No. 714,828 published Sep. 1, 1954 (Farbenfabriken Bayer); U.S. Pat. No. 2,717,237 issued Sep. 6, 1955 (Rempel); Japanese Patent application No. 81-158883, published Dec. 7, 1981; and U.S. Pat. No. 4,542,008 issued Sep. 17, 1985 (Capuano et al.).
U.S. Pat. No. 5,084,149 (issued Jan. 28, 1992 to J. J. Kaczur et al. discloses an electrochemical process for manufacturing chlorine-free chlorine dioxide from a diluted alkali metal chlorite solution containing a conductive salt additive in a single step. The electrolytic cell used contains a porous flow-through anode and a cathode separated by a suitable separator.
U.S. Pat. No. 5,092,970 and 5,106,465 (issued Apr. 21, 1992 to J. J. Kaczur et al.) discloses a process for electrolytically producing an aqueous solution of chlorine dioxide in a electrolytic cell having an anode compartment, a cathode compartment, and at least one cation ion exchange compartment between the anode and cathode compartments. An aqueous solution of an alkali metal chlorite is fed to the ion exchange compartment. The anolyte in the anode compartment is electrolyzed to generate hydrogen ions. The hydrogen ions are passed from the anode compartment through the membrane into the ion exchange compartment to displace alkali metal ions and produce an aqueous solution of chlorine dioxide. The alkali metal ions from the ion exchange compartment are passed into the cathode compartment.
In the '465 patent, the use of additives or activators in the chlorite feed solution is disclosed. The additives or activators promote more efficient conversion of chlorite to chlorine dioxide and suppress chlorate formation. Suitable additives include inorganic alkali metal salts and/or chlorides, phosphates, and sulfates and alkali metal tartrates and citrates.
U.S. Pat. No. 5,294,319 (issued Mar. 15, 1994 to J. J. Kaczur et al.) discloses a porous high surface area electrode particularly suitable for use in electrochemical processes.
A disadvantage of the above electrolytic processes is the production of the chlorine dioxide in the anode compartment of the generator so that the chlorine dioxide must be recovered from the anolyte by stripping with air or by some other appropriate means.
The generation and use of chlorine dioxide solutions poses a significant problem because the generation of chlorine-free chlorine dioxide is complex and requires a number of purification steps, including the stripping step discussed above and reabsorbtion of chlorine dioxide from a generating solution to a receiving solution. A stream of air is frequently used for this purpose; however, operation of such a process is hazardous if the chlorine dioxide concentrations in the air become high enough to initiate spontaneous decomposition. U.S. Pat. No. 4,683,039 (Twardowski et al.) discloses a purification method involving the use of a gas-permeable hydrophobic membrane. This purification method reduces the risk of chlorine dioxide decomposition but requires additional costly equipment.
The above problems were solved by employing a continuous electrochemical process and an electrolytic cell containing a porous flow-through anode. Chlorine-free chlorine dioxide was produced in a concentration of at least about 2 to about 10 grams per liter from dilute alkali metal chlorite solutions in a single step. This process and the cell are described in U.S. Pat. No. 5,158,658 issued Oct. 27, 1992 (Cawlfield et al.), the disclosure of which is incorporated herein by reference.